Whether you are a small or medium-sized enterprise looking for a miniature cooling tower, or a large industrial plant that requires multiple massive cooling towers, we have you covered in this guide. Designed to give you an extensive yet easy-to-follow view on the management of cooling towers, it covers the basics, as well as the most common best practices and procedures.
The Cooling Tower
Cooling Towers come in a variety of configurations which vary according to the erection site, the materials used, the airflow encountered and the type of draft used. These characteristics and other design factors are what distinguish one cooling tower design from another.
The Water Flow
In a typical cooling tower, the hot water enters a cooling tower through a pipeline system using strong pumps.
Most of the water exits through evaporation, which cools down the system. Along with evaporation, a small amount of water may be carried out as mist or droplets of water, but this is comparatively a small amount.
Since the cooling water is recirculated several times, certain salts or compounds (particularly calcium and sulfates) may build up as water is evaporated and they are left behind. Evaporation increases the concentration of the water and can lead to problems. Therefore, a portion of the concentrated water is drained, and fresh water is added. This process is known as blowdown.
Using Water Efficiently
Any water lost from the cooling tower must be replaced. Even in small or medium sized cooling towers, the system must constantly add water. The amount of water added equals the sum of the water lost through three different means: evaporation, blowdown and drift, i.e. as mist or droplets.
For the blowdown water, a key parameter to consider is the cycle of concentration. The cycle of concentration or concentration ratio is the ratio of concentration of dissolved solids in the blowdown water compared to the compensation water. It is also calculated in several other ways, including the ratio of volume of make-up water to the volume of blowdown water.
The ratio of water must be kept high to reduce the make-up or compensation of water. The higher concentration of water can lead to corrosion and build-up of scaling in the pipeline systems. If not kept under control, this can cause serious problems, leading to the replacement of one or more cooling tower parts.
Therefore, within the constraints of your cooling system, the cycle of concentration must be kept high without compromising the system. Raising the value of the concentration cycles from three to six can lower the make-up water by 20% and the blowdown by almost 50%.
Other procedures to use water efficiently include:
- Using recycled water from the plant or facility
- Pretreating water to reduce concentration of compounds in the first place
- Using water from municipality sources like high-quality treated wastewater
- Using air condensators which capture the low-temperature mist and return it as water to the cooling system
By increasing the concentration ratio and using cheaper sources of water, the cost for compensation or make-up water can be reduced. Often the water from the cooling system must be treated, and a third party is involved. In this case, it is important to find the right treatment vendor since water efficiency is your priority.
A good management tip is to determine the tradeoff between the cost of the treatment chemicals and water costs. Sometimes, depending on your vendor relationships and location, you can negotiate and obtain significant savings in treatment chemical costs.
Reducing Scaling and Corrosion
Since the high concentration of certain substances in the re-circulated water can cause scaling and corrosion of the water system, most of the cooling systems have two to four cycles of concentration. However, complementing the system with some additional measures to counteract the negative effects can increase the cycles to six or more.
The maximum number of cycles depends highly on the chemistry of the water used and the system it flows in. For example, some magnesium and calcium salts have limited solubility and may limit the increase in cycles.
A method to reduce scaling and corrosion due to concentration is to add conditioning chemicals like organophosphates for scaling and aromatic azoles to inhibit corrosion. However, care must be taken that these chemicals do not cause environmental problems, and thus more expense, when the water is drained. For example, conditioning chemical molybdates are prohibited if you discharge water to the sanitary sewer in the JEA service area, so make sure you are abreast of local, state and national regulations for hazardous emissions.
Alkaline elements in the water (bicarbonates and carbonates) contribute to the scaling in the system. To counter this, acids are added in safe amounts to the cooling water, which converts these scaling elements into more soluble forms, thus inhibiting their scaling properties. Biological fouling inhibitors are typically also added to the cooling water.
Measurements and Recording
As with any project or process, it is important to make certain measurements to ensure everything is operating optimally. When it comes to cooling towers, it is important to measure and track the performance of the cooling system. Tracking performance not only indicates problems but also can indicate the lifetime of the system and suggest it is time to replace cooling towers or parts of the cooling system.
A conductivity controller can be used to measure the conductivity of the water through the cooling system. As the concentration of the salts and other compounds rises, the conductivity measured in micro Siemens per centimeter (µS/cm) increases. This measurement can be used to find out the exact threshold at which the blowdown water must be discharged.
By accurate measurements, instead of having the water go a set number of cycles, the water is only drained when it exceeds a particular conductivity. This drainage can increase the maximum number of cycles of concentration.
Flow meters are also a great investment for cooling towers. To manage a cooling tower, you must have accurate measurements of the values in the water flow equation. The water flow equation is:
Make-Up = Evaporation + Blowdown + Drift
By installing flow meters on the make-up water line and the blowdown line, the ratio of make-up flow to the blowdown flow can be accurately calculated.
If acids or similar chemical conditioners are added to the water, it is also important to measure the pH of the water. High acidity can be detrimental to the cooling system. Acids are added at a high flow point to facilitate mixing with alkaline elements. The amount of all added chemicals must be measured and monitored, to not only save the cooling system, but also to not exceed the limits set by EPA and other organizations.
It is also critical to inspect the air handler coils, particularly in small or medium-sized cooling systems where regular inspection and maintenance is not mandatory. Through use and flow of humid and hot water, the coils can become dirty and fouled.
In this case, the impaired coils may put extra pressure on the chilled water system to maintain previous conditions. This pressure not only means that the coils may require more (electrical) energy but also results in more water being used. Tower fans are typically maintained to maximize the evaporation rate and thus help maximize cooling.
The water flow equation can help identify problems with the water cooling system. For example, the ratio of the make-up water to the blowdown water can be calculated from the measurements of the water flow in those lines.
You can then compare this ratio to the conductivity of both water lines. Essentially, both ratios should be close to (or ideally match) the cycles of concentration. If there is discrepancy in these ratios, this indicates a problem.
The fault can be in the measurements of the meters, and therefore the first step is to check and validate both the flow meters and the conductivity meters. If the measurements are correct, then the lines must be checked for leakage or unaccounted draw-off of water. The cooling tower must be operating near the targeted cycles of concentration given by the ratios.
If this is not the case, leakage testing must be performed. Several of the components of cooling towers, including the conductivity controller, make-up water fill valve and the blowdown valve, may have to be checked as well.
Leakage can also be indicated if there is mismatch in the water flow equation. The values of evaporation, drift, and blowdown must equal the make-up water. It is important to track and record the measurements from the cooling tower system. Even if the ratios are close to the targeted cycles of concentration, the trend of the measurements can indicate the deterioration in the performance of the cooling system.
The next aspect of management of cooling towers and water systems is the retrofit options. The first option to consider is targeted towards large-sized cooling towers. Often the water source for these towers comes from a natural body of water, which can lead to ecological issues for large cooling towers.
One of the problems associated with taking water from natural bodies is the high amount of silt and suspended particles in the water stream. The EPA suggests certain measures must be followed in these cases
To counter such problems, installation of a side-stream filtration system, which can filter the water from silt and suspended particles, is recommended. It helps to prolong the life of the tower and the water used.
To limit or inhibit biological growth, the sunlight reaching the cooling tower can be reduced. Much of the biological growth thrives under the humid conditionsand sunlight. By limiting the amount of sunlight, and inhibiting photosynthesis, biological growth including algae can be significantly reduced.
For large tower systems (> 100 tons), an automated feed system is also recommended. This system performs real-time chemical monitoring and controls the chemical use in the water. The chemical amounts are optimized to increase the concentration cycles while limiting problems like scaling, corrosion and biological growth. For smaller systems, consider using one of the other options.
Water treatment can be very expensive, particularly for hard water from the cooling system. Cascaded water treatment is effective, and other options include ozonation and ionization. Using these processes may significantly reduce the costs associated with a water treatment vendor.
However, before deciding on in-house treatment, realize these processes may also impact the life of the cooling system and must be installed and used with care. Water softening systems are also a good choice for reducing the hardness caused by calcium and magnesium compounds and can increase the maximum number of concentration cycles.
Management for Replacement
Through measurements of different parameters of the cooling towers and cooling system, and tracking these numbers, you can recognize when the cooling tower needs to be replaced. Whenever the cooling towers or parts must be replaced, always consult an expert before making a final decision because of the number of mechanisms involved and the technological advancement of parts and systems.
Many of the new designs can operate at higher concentration cycles and are more efficient at transferring the heat from the water to the air. Cooling towers, even the miniature ones typically installed on rooftops for small plants or data centers, are expensive. New designs must be compatible with the existing pumps and other cooling system equipment. However, newer systems require less management, and many of the modern designs include auto-metering, flow and heat transfer information and other useful data.
Utilizing appropriate resources, including consultation with vendors and experts in the field, will save you time and money. The process can be complicated and includes meeting EPA, State and National regulations.
Cooling towers must be selected and installed considering process requirements, compatibility with available pumps and cooling system, reliability of the chemical nature of the water source, ease of discharge for the used water and feasibility of treatment.
All these factors and more go into the installation, maintenance, operation and replacement of cooling towers.